In digital systems it is frequently necessary to process a signal at different sample rates. For example, a multimode audio processor may be arranged in a first mode to process a single audio signal occurring at a sample rate F, in a second mode to process two audio signals in time division multiplex fashion where each signal is processed at a rate F/2, and n a third mode to process N audio signals in time divisions multiplex fashion where each signal is processed at a rate F/N. Assuming each of the N audio signals occur at a sample rate F and include a spectrum of information including frequencies approaching F/2 (the Nyquest sampling limit), the N audio signals must be filtered to reduce the information spectrum to less than F/2N to prevent aliasing, and then subsampled to a rate F/N before time division multiplexing for application to the processor. The bandwidth of the respective antialias filters must be adjustable in accordance with the number N of signals to be time division multiplexed.
As a further example, consider a television TV receiver having a picture-in-picture processor which can simultaneously display a selectable number of reduced sized images. The processor may display four images each of which is compressed to occupy 1/4 of the display screen, nine images each of which is compressed to occupy 1/9 of the display screen, 16 images each of which is compressed to occupy 1/16 of the display screen, etc. Assume that the video signals representing each of the images is sampled to include the full video bandwidth of a non-compressed image. In this instance the picture-in-picture processor will include a selectably adjustable antialias filter or filters to limit the information bandwidth of each of the images in accordance with the selected compression factor.
Programmable digital low pass filters for adjusting the information passband of signals in digital format are known. Typically these filters take the form of output weighted finite impulse response filters with provision for changing the weighting factors. However, such filters tend to become relatively complicated if the filter is to accommodate even a relatively narrow range of different bandwidths.
It is also known to form programmable low pass filters as infinite impulse response or recursive filters. These filters also include weighting elements which are selectably programmable to define the signal passband.
Nominally, in both of the foregoing types of filters, a fixed set of weighting coefficients are applied to the weighting elements to define a particular passband. Fling et al. in U.S. Pat. No. 4,656,516 describe, with reference to FIG. 3A therein, a fixed bandwidth recursive type antialias filter which employs a reciprocating sequence of weighting coefficients. The Fling et al. filter forms a pseudoaverage of mutually exclusive groups of N samples wherein each of the samples in each group is weighted by a different coefficient. This filter is particularly advantageous in that it requires only one weighting element and therefore is very parts efficient.
It is an object of the present invention to provide a variable bandwidth filter embodied in a subsampling system which (a) employs a recursive filter arrangement; (b) utilizes reciprocating sets of weighting coefficients; and (c) is simply programmable using for example the reciprocal of the desired subsampling ratio.